The "padlock" icon after a mobile update occurs when you update your status from a mobile. You cannot remove it as such. If you want it to not appear, you should be logged on to www.facebook.com from your mobile and see facebook in its real form instead of m.facebook.com. If you are using an application to be logged into facebook, facebook will know that you are using it on mobile and place the icon. If you are updating your facebook status via sms, then also facebook will know and place the icon. The only way is to be logged into www.facebook.com from a mobile browser.
And about some of your friends being able to see your status updates and some not - that's insane, I do not think it is possible. The only thing that I can come up with is that you have messed up your privacy setting on facebook or may be the app you use to log on to facebook from mobile allows you to set privacy settings separately and you have messed it up there.